How is (1/2)V_id expressed in this circuit?

[song youngill]

Homework Statement

instrumentation amplifier in ltspice

Relevant Equations

vin=1/2(vid)+1/2(vcm)

As you can see in the picture, 1/2vid is entered in each of Vin1 and Vin2 on the left. Assuming Vcm is 0, I need to configure this circuit with ltspice, but I am not sure how to configure this vcvs. I'd appreciate it if you could tell me how.

 

[.Scott]

Your diagram does not show any Vin - neither Vin1 nor Vin2.
That said, I would guess that Vin1 and Vin2 are the "+" nodes of the top and bottom op amps, respectively.
If that is the case, then neither Vin is $V_{CM}/2 + V_{ID}/2$.

One other thing. Those diamonds are most commonly a symbol for a current source. But they are labelled with a voltage value ($(1/2) V_{ID}$). So I take them as controlled voltage sources.

 

[song youngill]

Your diagram does not show any Vin - neither Vin1 nor Vin2.
That said, I would guess thaVin1 and Vin2 are the "+" nodes of the top and bottom op amps, respectively.
If that is the case, then neither Vin is $V_{CM}/2 + V_{ID}/2$.

sorry. vin1 is upper amp's + input and vin2 is lower amp's + input. The does 1/2vid can't be configured as vcvs in ltspice?

 

[.Scott]

sorry. vin1 is upper amp's + input and vin2 is lower amp's + input. The does 1/2vid can't be configured as vcvs in ltspice?

Oh, I see. Those diamonds are supposed to be Voltage Controlled Voltage Sources (VCVS's).
I am not familiar with ltspice. Perhaps you would need to actually draw out the voltage divider.
But do you realize that the value you specified before, $V_{CM}/2 + V_{ID}/2$, is not a voltage level used in this circuit?

 

[.Scott]

OK. I just checked out LTSpice.
It looks like you need to create two more voltage sources - each $V_{ID}$. Then use those with "e" components (VCVSs), each with a gain value of 0.5.
That should do the trick.

 

[song youngill]

OK. I just checked out LTSpice.
It looks like you need to create two more voltage sources - each $V_{ID}$. Then use those with "e" components (VCVSs), each with a gain value of 0.5.
That should do the trick.

You can probably replace it with an equivalent independent voltage source. I need to observe the vo value according to the change of the vid, so I hope I can solve the vid.

 

[Tom.G]

Here is a bit more detail.

V_ID is Voltage, Input, Differential (or differential input voltage)

The simulation circuit shows that the V_CM (Common Mode Voltage) affects both input OpAmps.

Tha advantage of using the two VCVS's is you need to vary only one input voltage (independent voltage source) which then controls the two VCVS's and both differential inputs to the circuit.

Have Fun!

Cheers,
Tom